Enable shellcheck to run for select scripts

Enable shellcheck to run on zed scripts,
paxcheck.sh, zfs-tests.sh, zfs.sh, and zloop.sh.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5812

Makefile.am

@@ -47,11 +47,14 @@ cstyle:
 
 shellcheck:
 	@if type shellcheck > /dev/null 2>&1; then \
-		(find ${top_srcdir} -type f -name '*.sh.in' -o -type f \
-		 -name '*.sh'; find etc/init.d/zfs*.in -type f) | \
+		shellcheck --exclude=SC1090 --format gcc scripts/paxcheck.sh \
+			scripts/zloop.sh \
+			scripts/zfs-tests.sh \
+			scripts/zfs.sh; \
+		(find cmd/zed/zed.d/*.sh -type f) | \
 		 grep -v 'zfs-script-config' | \
 		 while read file; do \
-			shellcheck --format gcc "$$file"; \
+			shellcheck --exclude=SC1090 --format gcc "$$file"; \
 		 done; \
 	 fi